Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Some of the more popular bar code symbologies include: Uniform Product Code (UPC), typically used in retail stores sales; Code 39, primarily used in inventory tracking; and Postnet, which is used for encoding zip codes for U.S. mail. Systems that read and decode bar codes employing charged coupled device (CCD) or complementary metal oxide semiconductor (CMOS) based imaging systems are typically referred to hereinafter as imagining systems, imaging-based bar code readers or bar code scanners.
Bar code reader systems electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. The characters are then typically represented in digital form and utilized as an input to a data processing system for various end-user applications such as point-of-sale processing, inventory control and the like. Scanning systems of this nature have been disclosed for example, in U.S. Pat. Nos. 4,251,798; 4,369,361; 4,387,297; 4,409,470; 4,760,248; and 4,896,026, all of which have been assigned to the assignee as the instant application.
Imaging readers employing CCD type technology typically employ an illumination system to flood a target object with illumination from a light source such as a light emitting diodes (LED) in the reader, and each CCD cell is sequentially read-out to determine the individual spacing in the bar code. Imaging systems include CCD arrays, CMOS arrays, or other imaging pixel arrays having a plurality of photosensitive elements or pixels. Light from the light source or LED is reflected from a target object, such as a bar code. The reflected light is then focused through a lens of the imaging system onto the pixel array, the target object being within a field of view of the lens. An analog-to-digital converter then digitizes output signals generated from the pixels of the pixels array. Decoding circuitry of the imaging system then processes the digitized signals and attempts to decode the imaged bar code.
The working range of prior art CCD and CMOS imaging-based bar code reader may be limited by the illumination system and ambient lighting. As the distance between the target object and the illumination system increases, the intensity of the illumination at the target object decreases. Reflected illumination from the target object is focused on the pixels of the CCD pixel array. Depending on the characteristics of the CCD pixel array, some minimal level of reflected illumination is necessary to successfully capture the image of the target object and decode the indicia encoded in the captured image. Thus, as the target object distance is increased, the intensity required from the illumination system increases thereby increasing the power requirements of the illumination system. Since most readers are battery powered, minimizing power requirements to increase working time between recharges is of great importance.
Additionally, the focusing lens of the imaging assembly is characterized by an angular field of view in the horizontal and vertical directions. Therefore, as the target object distance increases, the portion of the field of view occupied by the target object necessarily decreases. Due to the fact that the effective illumination is decreasing at a factor of the distance to the target object squared, greater illumination intensity from the illumination system will be required to successfully image the target object. If the reader utilizes a single illumination system for both short and long working range-imaging applications, a high intensity illumination system will be required to image target objects at longer working ranges. Such a high intensity illumination system will waste power by illuminating the entire field of view unnecessarily when only a subset of the field of view (FOV) is necessary.
What is need is an imaging-based reader, such as an imaging-based bar code reader, including an imaging system for imaging a target object in a field of view of the imaging system and an illumination system that provides for an increased effective range of the imaging system. What is also needed is an imaging-based reader that provides an illumination system comprising a first illumination pattern for imaging a target object within a first imaging range and a second illumination pattern to be used to conjunction with the illumination system, to extend the effective imaging range of the imaging system to a second imaging range greater than the first imaging range.